Module ngx_http_limit_conn_module

Example Configuration
Directives
     limit_conn
     limit_conn_log_level
     limit_conn_status
     limit_conn_zone
     limit_zone

The ngx_http_limit_conn_module module is used to limit the number of connections per the defined key, in particular, the number of connections from a single IP address.

Not all connections are counted. A connection is counted only if it has a request processed by the server and the whole request header has already been read.

Example Configuration

http {
    limit_conn_zone $binary_remote_addr zone=addr:10m;

    ...

    server {

        ...

        location /download/ {
            limit_conn addr 1;
        }

Directives

Syntax: limit_conn zone number;
Default:
Context: http, server, location

Sets the shared memory zone and the maximum allowed number of connections for a given key value. When this limit is exceeded, the server will return the 503 (Service Temporarily Unavailable) error in reply to a request. For example, the directives

limit_conn_zone $binary_remote_addr zone=addr:10m;

server {
    location /download/ {
        limit_conn addr 1;
    }

allow only one connection per an IP address at a time.

In SPDY, each concurrent request is considered a separate connection.

When several limit_conn directives are specified, any configured limit will apply. For example, the following configuration will limit the number of connections to the server per a client IP and, at the same time, the total number of connections to the virtual host:

limit_conn_zone $binary_remote_addr zone=perip:10m;
limit_conn_zone $server_name zone=perserver:10m;

server {
    ...
    limit_conn perip 10;
    limit_conn perserver 100;
}

These directives are inherited from the previous level if and only if there are no limit_conn directives on the current level.

Syntax: limit_conn_log_level info | notice | warn | error;
Default:
limit_conn_log_level error;
Context: http, server, location

This directive appeared in version 0.8.18.

Sets the desired logging level for cases when the server limits the number of connections.

Syntax: limit_conn_status code;
Default:
limit_conn_status 503;
Context: http, server, location

This directive appeared in version 1.3.15.

Sets the status code to return in response to rejected requests.

Syntax: limit_conn_zone key zone=name:size;
Default:
Context: http

Sets parameters for a shared memory zone that will keep states for various keys. In particular, the state includes the current number of connections. The key can contain text, variables, and their combination. Requests with an empty key value are not accounted.

Prior to version 1.7.6, a key could contain exactly one variable.

Usage example:

limit_conn_zone $binary_remote_addr zone=addr:10m;

Here, a client IP address serves as a key. Note that instead of $remote_addr, the $binary_remote_addr variable is used here. The $remote_addr variable’s size can vary from 7 to 15 bytes. The stored state occupies either 32 or 64 bytes of memory on 32-bit platforms and always 64 bytes on 64-bit platforms. The $binary_remote_addr variable’s size is always 4 bytes. The stored state always occupies 32 bytes on 32-bit platforms and 64 bytes on 64-bit platforms. One megabyte zone can keep about 32 thousand 32-byte states or about 16 thousand 64-byte states. If the zone storage is exhausted, the server will return the 503 (Service Temporarily Unavailable) error to all further requests.

Syntax: limit_zone name $variable size;
Default:
Context: http

This directive was made obsolete in version 1.1.8 and was removed in version 1.7.6. An equivalent limit_conn_zone directive with a changed syntax should be used instead:

limit_conn_zone $variable zone=name:size;